Under some circumstances smoking articles (e.g., cigarettes) may ignite fire-prone substrates if the article is laid on or accidentally contacts such substrate. Therefore, a cigarette prepared from a wrapper which diminishes the ability of the article to ignite a substrate may have the desirable effect of reducing cigarette-initiated fires. Furthermore, a wrapper that concurrently confers on the cigarette the ability to free burn in a static state and reduced IP character allows a beneficial reduction in the tendency of the article to ignite fire-prone substrates while maintaining consumer acceptability. Other factors affecting consumer acceptability include product appearance as well as pleasing and consistent wrapper and ash character. Moreover, it is important that the construction of the smoking article exhibits a reasonable shelf-life while maintaining reduced IP.
It has been determined that cigarette wrapper porosity characteristics may contribute to both the reduced IP and free burn properties for a cigarette. Porous substrates and membranes have utility in a wide variety of applications, most of which involve selective flow of a fluid, gas or particulates through the pores. For example, separations of particulate matters from gases or liquids are common commercial applications of microporous membranes. Furthermore, enhanced separation of homo-, heterogeneous mixtures can be achieved by modification of the physico-chemical characteristics of membranes. Such application of enhanced separations are found in electrochemical cells (e.g. batteries, fuel cells, sensors, and capacitors) wherein membranes are used to separate electrodes while simultaneously allowing ions to transport. In these cases, the physical dimensions of pores and the surface chemistry are tailored to optimize performance. To that end, a cigarette paper can be thought of as a flexible membrane that surrounds a tobacco column, allowing gas diffusion in and out of the column during burning while holding/retaining the finely divided tobacco in a rod form.
Many methods exist to fabricate membranes of well-defined pore structure of membranes. Those known in the arts include (i) laser drilling of holes that yields perforated, straight-thru, and/or non-torturous holes in the filter membranes; (ii) evaporation of dissolved gases in a melted or reactive polymer, followed by chemical, mechanical or thermal breakage of cell walls to produce open cell foams; (iii) addition and activation of chemical blowing agents followed by chemical, mechanical or thermal breakage of cell walls to produce open cell foams; (iv) addition of soluble particles at high concentration in the polymer, followed by dissolution of the soluble particles to produce filter membranes with pores that match their original particle sizes and locations; (v) addition of plasticizer to a high concentrated polymer, followed by extraction of this plasticizer by a low-boiling solvent to produce an open cell battery separator; (vi) compression of polymer particles within a liquid medium causing bonds to form between such particles, followed by stretching of the polymer film either uniaxially or biaxially to produce gas/liquid separation membranes; (vii) slitting of a polymer film followed by lateral stretching; (viii) sol-gel, internal phase inversion followed by evaporation of solvent and production of an open-cell membrane; (ix) formation of patterned and collapsible porous structures yielding semi-porous membranes; (x) a sol-gel type, external phase inversion, followed by addition of an external to the polymer solution producing an open cell filter membrane. In similar methodology, cigarette papers can be manufactured to specific porosity using well-known paper making processes that result in specialty papers of a wide range of porosities for specific product requirements.
Chemical modification of the porous membranes to enhance specific separation properties is also known in the art, which teaches different approaches to modify semi-porous membranes during or after fabrication. The corresponding chemical processes include but are not limited to (1) modification of the surface of semi-porous membrane to make it either hydrophobic, or hydrophilic, or possessing the preferential affinity to specific chemical functional groups, therefore enabling selective retention; (2) UV radiation; (3) plasma radiation; (4) microwave radiation treatment.
The prior art teaches methods of fabrication or post-treatment modification of a semi-porous membrane to alter its porosity for controlling and reducing its gas permeability during use. For example, it is known to modify porous paper used in cigarette products by applying a starched based coating/layer by gravure techniques. The purpose of the coating is to create bands or regions that reduce the gas permeability of the paper substrate. Subsequently, the reduced oxygen flow in the coated regions imparts an ignition propensity to the article. In this case the pore structure of the membrane (paper) is reduced from the original base paper and remains static during use. Besides adding paper conversion costs to cigarette fabrication, this method is normally limited to off-line implementation because it requires drying the paper prior to use in cigarette making in addition to negatively affects the user experience if higher ignition propensity is desired.